One of the scarcest resources in current and future wireless communication systems for providing more capacity and higher data rates is available frequency spectrum. A possible way forward to meet the demands for more spectrum is to use the limited spectrum more efficiently, e.g. through spectrum sharing. This means that different systems are allocated to the same spectrum, the systems possibly being of completely different kinds (e.g. a radar system and a terrestrial mobile communication system). To be able to share spectrum, the interference from the other systems must be managed in a proper way.
In current mobile communication systems, interference from other systems is not dealt with in any particular way. It is handled as thermal noise or intra-system interference, and hence the co-existence with other systems within the same spectrum is virtually impossible. A special case will occur if the interference is of a special character, such as pulsed radar interference. In this case the interference will only affect parts of the information transmitted in the interfered system. Hence, it may be possible for two systems to share the same frequency spectrum, or adjacent frequency spectra, in some cases.
Due to the fact that interference affecting reference symbols, control signaling, synchronization symbols and user data affects the system differently, a strong interfering pulse from e.g. a radar system, though very short in time, might severely impact the performance of the system, by affecting the synchronization symbols, reference symbols and/or control signalling. This is because achieving synchronization is a necessary first step, reference symbols are assumed to be representative for the data and control signalling is used for decoding data.
As a specific example, one may consider a situation where an orthogonal frequency-division multiplexing (OFDM) symbol containing only user data, but no reference symbols (pilots), is affected. It can be shown that with the usual demodulation methods, the affected data symbols will generate soft values that are incorrect, but in spite of that have very large magnitude. This may result in an entire code block being destroyed, even though only a relatively small fraction of the symbols belonging to the code block are affected, and even if strong channel coding is being used. The short interfering pulses may be more or less periodic, or may have no periodicity at all. Since the short radar pulses may in a realistic case appear in the order of once per code block, especially if they are periodic, this may be a severe issue.